During the past year investigations have been directed toward relating the role of the regional lymph node (RLN) to the production of macophages and the cytotoxicity of such cells derived as a result of culture of bone marrow cells (CMAs). It was observed that not only is there decreased macrophage colony production when RLNs are absent, but cytotoxicity of the resultant CMAs is reduced. In additional experiments it was observed that transfer of RNCs to normal mice (i.v.) resulted in the production of cytotoxic CMAs in such animals. It was also observed that peripheral blood granulocytes derived from the same myeloid stem cells as the CMAs also displayed cytotoxicity which was specific to the immunizing tumor. Other investigations revealed that (a) serum inhibition of cytotoxicity increased in direct relationship to the duration of tumor growth in mice from which the serum was derived and (b) the cytotoxicity of RLNCs decreased with duration of tumor growth and inhibition of the cytotoxicity of such cells increased, i.e., such cells were less cytotoxic and were inhibited to a greater degree by the same serum. Studies to elucidate the temporal relationship of the occurrence of cytotoxicity in RLNCs, BMCs and macrophages revealed that BMCs and CMAs displayed a significant degree of cytotoxicity by the second day of tumor growth, whereas RLNCs achieved such status on the third day and demonstrated less cytotoxicity than did the other cell types. Relation of nodal histologic findings to cytotoxicity suggested that the pyroninophilic elements may have been more closely related to cytotoxicity than to tumor immunity per se. During the coming year efforts will be directed toward determining whether RLNs are of importance for the maintanence of CMA cytotoxicity and toward ascertaining the macrophage content of tumors in treated and untreated animals. The degree of macrophage content of primary tumors will be related to dissemination and metastases.